The invention relates generally to stents, which are endoprostheses implanted into vessels within the body, such as blood vessels, to support and hold open the vessels, or to secure and support other endoprostheses in the vessels.
Various stents are known in the art. Typically stents are generally tubular in shape, and are expandable from a relatively small, unexpanded diameter to a larger, expanded diameter. For implantation, the stent is typically mounted on the end of a catheter, with the stent being held on the catheter at its relatively small, unexpanded diameter. By the catheter, the unexpanded stent is directed through the lumen to the intended implantation site. Once the stent is at the intended implantation site, it is expanded, typically either by an internal force, for example by inflating a balloon on the inside of the stent, or by allowing the stent to self-expand, for example by removing a sleeve from around a self-expanding stent, allowing the stent to expand outwardly. In either case, the expanded stent resists the tendency of the vessel to narrow, thereby maintaining the vessel""s patency.
Some examples of patents relating to stents include U.S. Pat. No. 4,553,545 to Maass et al.; U.S. Pat. No. 4,733,665 to Palmaz; U.S. Pat. Nos. 4,800,882 and 5,282,824 to Gianturco; U.S. Pat. Nos. 4,856,516, 4,913,141, 5,116,365 and 5,135,536 to Hillstead; U.S. Pat. Nos. 4,649,922, 4,886,062, 4,969,458 and 5,133,732 to Wiktor; U.S. Pat. No. 5,019,090 to Pinchuk; U.S. Pat. No. 5,102,417 to Palmaz and Schatz; U.S. Pat. No. 5,104,404 to Wolff; U.S. Pat. No. 5,161,547 to Tower; U.S. Pat. No. 5,383,892 to Cardon et al.; U.S. Pat. No. 5,449,373 to Pinchasik et al.; and U.S. Pat. No. 5,733,303 to Israel et al.
U.S. Pat. No. 4,553,545 to Maass et al. (the xe2x80x9cMaass ""545 patentxe2x80x9d), the disclosure of which is hereby expressly incorporated by reference into this application, shows various stents in the form of coiled springs. FIGS. 1 through 7 of that patent illustrate coiled spring stents formed of helically wound wire wherein the diameters of the stents are contracted and expanded by rotating the spring ends. Such coiled spring stents are very flexible, such that they can be tracked easily down tortuous lumens and such that they conform relatively closely to the compliance of the vessel after deployment. However, while these stents are very flexible, they also lend relatively unstable support after expansion. The individual windings of the coil may move relative to each other, causing sometimes significant gaps between adjacent windings, which could cause significant portions of the vessel wall to be left unsupported. Also, the windings of the coil may bend or tilt somewhat, potentially obstructing and seriously compromising the lumen. FIG. 10 of the Maass ""545 patent illustrates an example of tilted windings in these coiled spring stents.
The Maass ""545 patent discloses various mechanisms designed to address the instability of these coiled spring stents. For example, FIGS. 11 through 14 show the use of rigidifying devices in the form of axial support members that extend along a side of the stent and maintain the relative positioning of the windings. FIG. 22 shows a stent constructed of a coiled band, wherein the band has openings in it so that it takes the shape of a ladder. FIG. 23 shows another coiled ladder stent, wherein the ladder is formed by two wires attached to each other by transverse elements. Each of these coiled ladder stents provides improved stability when compared to the single strand coiled spring stents.
Despite these modifications, one problem with each of the coiled spring and coiled ladder stents disclosed in the Maass ""545 patent is that expansion of the stent results in an unwinding of the coil. This unwinding causes twisting of the stent, including rotation of the stent ends, which is potentially harmful to the vessel wall. In addition, the expansion and twisting causes the number of individual windings to lessen, resulting in less windings to support the vessel wall. The reduction in windings also means either that the length of the stent will foreshorten significantly, in order to maintain the spacing of the windings, or that the spacing between the windings will increase significantly, in order to maintain the length of the stent, or in some instances a combination of both. The foreshortening results in less lengthwise coverage of the vessel wall in the deployed stent as well as lateral movement during deployment which may be harmful to the vessel wall. The increase in the spacing between windings could result in significant portions of the vessel wall being left unsupported. Both are potential disadvantages of the coiled spring and coiled ladder stents disclosed in the Maass ""545 patent.
U.S. Pat. Nos. 4,886,062 and 5,133,732 to Wiktor (the xe2x80x9cWiktor ""062 patentxe2x80x9d and the xe2x80x9cWiktor ""732 patentxe2x80x9d), the disclosures of which are hereby expressly incorporated by reference into this application, show stents formed of wire wherein the wire is initially formed into a band of zig-zags forming a serpentine pattern, and then the zig-zag band is coiled into a helical stent. The stents are expanded by an internal force, for example by inflating a balloon. Another example of a similar coiled zig-zag stent is the Crossflex stent marketed by Cordis Corporation.
The coiled zig-zag stents that are illustrated in FIGS. 1 through 6 of the Wiktor ""062 and ""732 patents are very flexible, but, again, they are relatively unstable. The Wiktor ""732 patent discloses alternative constructions of these coiled zig-zag stents to address their instability. In one example, illustrated in FIG. 7 of that patent, a straight longitudinal wire extends along a side of the stent and is connected to the windings to fix them relative to each other. In another example, illustrated in FIG. 8 of that patent, in various locations around the helix of the stent, a bend in the zig-zag wire is made longer than other bends, so that it can reach and hook around a bend in an adjacent winding of the helix. Each of these constructions results in increased stability of the stent, but each also results in some reduction in the flexibility of the stent.
An object of the invention is to provide a stent that is longitudinally flexible such that it can easily be tracked down tortuous lumens and does not significantly change the compliance of the vessel after deployment, wherein the stent is relatively stable so that it avoids bending or tilting in a manner that would potentially obstruct the lumen and so that it avoids leaving significant portions of the vessel wall unsupported.
Another object of the present invention is to provide a stent that has little or no twisting or rotation of its ends upon expansion, and that also undergoes little or no foreshortening upon expansion and simultaneously does not result in significant gaps being created between adjacent windings of the stent upon expansion.
In accordance with one embodiment of the invention, a stent is configured as a coiled stent in which the coil is formed from a wound strip of cells, wherein the sides of the cells are serpentine. Thus, the stent is comprised of a strip helically wound into a series of coiled windings, wherein the strip is formed of at least two side bands connected to each other, for example by a series of cross struts. Each side band is formed in a serpentine pattern comprising a series of bends, wherein upon expansion of the stent, the bends of the side bands open to increase the length of each of the individual cells in the helical direction, thereby lengthening the strip in the helical direction to allow the stent to expand without any significant unwinding of the strip.
A serpentine coiled ladder stent according to the invention retains the flexibility associated with coiled spring stents, yet has windings which are relatively stable and insusceptible to displacement or tilt. A serpentine coiled ladder stent according to the invention thus provides continuous support of the vessel tissue without disadvantageously obstructing the lumen.
In addition, the serpentine coiled ladder stent substantially avoids the problems of twisting, end rotation, foreshortening and the creation of significant gaps upon expansion. When the serpentine coiled ladder stent is expanded, the outward radial force on the stent causes the bends in the serpentine sides to open up and become straighter, thereby causing the overall length of the strip in the helical direction to increase. By providing a serpentine strip that allows the strip itself to lengthen in the helical direction as the stent is expanded, the increase in the diameter of the stent is accommodated by a lengthening of the strip, rather than by an unwinding of the strip. Thus, the number of windings may be maintained, and rotation of the ends and foreshortening or the opening of gaps between windings can be substantially reduced or avoided. In fact, the serpentine coiled ladder stent may be constructed so that adjacent points on adjacent windings remain adjacent to each other as the stent is expanded. Thus, the two ends of the strip at the ends of the stent may be joined, for example by welding, to the respective adjacent windings, thereby creating smooth ends and assuring no relative rotation.